Tart rounded up "D.J. Tommy Boy" and opened the dance floor five hours early with an eclectic mix of blues, '80s tunes and heavy metal.
Feeling sorry for the fans who missed out on the last four blues acts because of the weather, Tart said, he shaved a buck off the price of hamburgers, hot dogs and appetizers.
"We just wanted to be accommodating," said Tart, who opened the bar in December with business partner Shane Kline.
Tart said the bar, which holds 150 people, was full the rest of the afternoon.
Around the corner at the Broad Axe, about 100 people filed in and the place stayed packed until 1 a.m., chef Maria DeVore said.
Local blues band Rudy & the Bluefish wasn't supposed to start playing until 10 p.m., but they came on about 5 p.m. and played nonstop with the help of some other area musicians.
"It was like a jam night," DeVore said. "This is the best Blues Fest we've ever had."
Burhans Station also saw a boost in business, but co-owner David Lookabaugh said it was hard to gauge whether it was due to the rain because this was the establishment's second Blues Fest.
"We got some people earlier probably than what we would have got them," he said.
Some downtown restaurants didn't fare as well as the bars.
Roccoco lost business because the party broke up hours before dinnertime and most people were too waterlogged to stay, manager Susie O'Boyle said.
Blues Fest organizers, who called off the day's activities due to the threat of lightning and high winds, were disappointed.
"We work on it all year. We're the last ones that want to cancel," said Karen Giffin, public information manager for the city of Hagerstown.
The festival bought rain insurance, but organizers won't know for about a week whether they will be able to collect the $40,000.
The policy only pays if a half-inch of rain was reported at Hagerstown Regional Airport between 6 a.m. and 4 p.m., said Sandy Mills, commercial lines underwriter at Keller-Stonebraker Insurance Inc.
The company uses results collected by the National Oceanic and Atmospheric Administration, which won't be available for about a week, she said.
According to local weather observer Greg Keefer, Hagerstown received .32 inches of rain all day, which would fall short of the threshold.
Rain held off Sunday for the final day of the Blues Fest, a free day of music in Hagerstown City Park.